Conventionally, 4-imino-oxazolidine-2-one derivatives having substituted aryl groups on the nitrogen atom in position 3 are known to have fungicidal activity (see, for example, Japanese Patent Publication No. 43800/1972); however, there have been no reports on the synthesis of 4-imino-oxazolidine-2-one derivatives having an isopropylidene group in position 5 of the oxazolidine ring as represented by the general formula (1) in the present invention and it is entirely unknown that such derivatives have a potent herbicidal activity.
The 4-imino-oxazolidine-2-one derivatives described in Japanese Patent Publication No. 43800/1972 are produced by addition and cyclization reactions of the corresponding isocyanate and cyanoalcohol. However, no reaction has yet been known that employs a cyanoalcohol having a double bond in the a position and, what is more, the introduction of an isopropylidene group utilizing the isomerization of the double bond is a reaction that is a key to the synthesis of the 4-imino-oxazolidine-2-one derivatives of the present invention.
EP-0241559B teaches that oxazolidinedione derivatives represented by the general formula (3) are useful as the active ingredient of the herbicide it claims and such derivatives may be produced by subjecting an aryl isocyanate (2) to an addition cyclization reaction with a 2-hydroxy-3-methyl-3-butenoic acid ester (4) according to the scheme shown below. However, this process requires a two-step reaction for preparing the 2-hydroxy-3-methyl-3-butenoic acid ester which is used as a starting material and it is necessary to use many auxiliary materials; what is more, the desired product cannot necessarily be obtained with satisfactory yield.
Thus, the conventional method which uses the 2-hydroxy-3-methyl-3-butenoic acid esters (4) not only involves a prolonged reaction process but also requires the use of many auxiliary materials in association with the individual steps and these and the additional problem of low yield make the method economically disadvantageous for the production of oxazolidinedione derivatives of the general formula (3) which are useful as the active ingredient of herbicides. Hence, it is desired to develop a more efficient production process. ##STR5## (where X.sup.1, X.sup.2 and X.sup.3 each have the same meaning as defined above; R' is a lower alkyl group).